Sera from asymptomatic seropositive patients display significant levels of neutralizing antibodies against HIV-1. These antibodies are group-specific, capable of neutralizing most or all of the broad spectrum of HIV-1 isolates, in spite of the genetic diversity among them. To explain these findings, we have searched for the site on the virus that is recognized by these .antibodies. We have used the HIV-1 plaque forming assay to detect the level of neutralizing activity in human sera after adsorbing the sera with a variety of recombinant viral proteins. If the neutralizing site is located on a given protein, then antibodies adsorbed with that protein will be depleted of neutralizing activity; this would be measured in our assay as increased viral survival in the presence of the adsorbed serum. Our initial findings used complete viral proteins: gpl6O, gp4l, and p24. We found that greater than 90% of neutralizing antibodies were adsorbed by gpl6O, but none were adsorbed by gp4l or p24. Now, we have extended these findings by adsorption on gpl2O and fragments. We find that the site is contained on full length gpl2O (amino acids 1-511) but not on the large fragment 1-471 or on 472-861, nor is it adsorbed by sequential adsorption on 1-471 followed by 472-861. This suggests that the site may be conformational, depending on both fragments, but contained on neither. Secondly, we find that these antibodies are not adsorbed on the gpl2O of SIV, but they are adsorbed on a hybrid gpl2O containing SIV sequences at the amino 2/3 and HIV-1 sequences at the carboxyl 1/3. We conclude that the predominant neutralizing epitope of HIV-1 is located on the carboxyl 1,,/3 of gpl2O and is conformational. It closely resembles the CD4 binding site and entirely excludes the V3 loop structure emphasized by others. Knowledge of this site may help in vaccine design or evaluation.